A fraction where the denominator (the bottom number) is a power of ten (such as 10, 100, 1000, etc).
You can write decimal fractions with a decimal point (and no denominator), which make it easier to do calculations like addition and multiplication on fractions.
Examples:
7/10 is a decimal fraction and it can be shown as 0.7
43/100 is a decimal fraction and it can be shown as 0.43
51/1000 is a decimal fraction and it can be shown as 0.051